(v6) Update old proxy version

[r0mant]

Update the isOldCluster logic to check remote proxies of version less than 6.0. Otherwise, after upgrading the root cluster it tries to set up the remote cluster cache to watch for the new database servers resource which produces the following error on the leaf:

two-auth     | INFO [RBAC]      Access to read db_server in namespace default denied to roles RemoteProxy,default-implicit-role: no allow rule matched. services/role.go:2083

With this fix, the error no longer shows, I see this on the root side which seems normal?

DEBU [RBAC]      Check access to role(Proxy) rc(leaf, labels=map[]) matchLabels=true, msg=matched, err=<nil> allow=map[*:[*]] rcLabels=map[] services/role.go:1305

#5721 backport.